In spraying articles with liquid such as paint or sealant, it is important to completely cover the article. In automatic systems, if there is a malfunction in the paint spray system, the articles may be insufficiently coated. If the malfunction is not detected, the articles may have to be manually painted or must be passed through the system a second time. This obviously results in inefficient use of relatively expensive equipment, and potentially expensive manual rework.
A typical malfunction of such a liquid spray system is that the nozzle or spray tip of a spray gun of the system becomes plugged or clogged. This clogging or plugging may be either partial or total.
Tip blockage detection has been accomplished by using a single pressure transducer or sensor as illustrated at 10 in FIG. 1. This system works on the principle that if the pressure increases at the nozzle 12 of a spray gun 14, the spray gun tip is blocked. The transducer 10 may be placed anywhere in the system close to the gun 14 remote from a source 15 of pressurized liquid. Pressure limits are set in the software contained within a controller 16 of the system to flag a high pressure condition. In addition to the pressure transducer 10, a flow meter 18 may optionally be located in a liquid supply line 20 of the system.
One problem with the single transducer approach is that such a system is very sensitive to material viscosity changes and incoming material pressure changes. This is because the system works on the principle that a change in pressure indicates a plugged spray orifice. Pressure changes due to changes of the material causes enough of a change in pressure to indicate a plugged tip or a low pressure condition, which may or may not be true.
U.S. Pat. No. 4,430,886 to Rood discloses a method and apparatus for sensing a clogged nozzle in a spray gun by placing a restricter in the liquid flow path upstream of a valve and a pressure transducer between the restricter and the valve for measuring the pressure drop when the valve is open.
U.S. Pat. No. 3,482,781 to Sharpe discloses a spray gun including a pressure gauge that directly reads the pressure of air applied to the nozzle.
U.S. Pat. No. 4,368,852 to Sharpe et al discloses a combination spray gun and pressure regulator for providing even pressure of the liquid supply to the gun.
U.S. Pat. Nos. 3,385,522, 4,181,017 and 5,096,120 to Kock, Markle and Luckarz, respectively, all disclose pressure regulated spray systems. The '120 patent discloses the use of one sensor associated with a recirculating line and a second sensor associated with a tapping line.